Vacuum IG units are known in the art. For example, see U.S. Pat. Nos. 5,664,395 and 5,657,607, the disclosures of which are both hereby incorporated herein by reference.
Prior art FIGS. 1-2 illustrate a conventional vacuum IG unit. IG unit 1 includes two spaced apart sheets of glass 2 and 3 which enclose an evacuated or low pressure space therebetween. Glass sheets 2 and 3 are interconnected by peripheral seal of fused solder 4 and an array of support pillars 5.
Pump out tube 8 is hermetically sealed by fused solder glass 9 to an aperture or hole 10 which passes from an interior surface of glass sheet 2 to the bottom of recess 11 in the exterior face of sheet 2. Recess 11 allows pump out tube 8 to be melted and sealed leaving a stub which does not protrude beyond the plane of the exterior face of sheet 2. When sheets 2, 3 and peripheral/edge seal 4 are in place, prior to the tube being sealed, a vacuum is attached to pump out tube 8 so that the interior cavity between sheets 2 and 3 can be vacuumed out to create a low pressure area. Chemical getter 12 may optionally be included within a machined recess 13 in one of the glass sheets to counteract any rise in pressure due to out-gassing from the glass.
Conventional vacuum IG units, with their fused solder glass peripheral seals 4, have been manufactured as follows when the upper glass sheet is slightly smaller in dimension than the lower sheet (not shown in FIGS. 1-2). Solder glass is initially deposited around the periphery of the IG unit in an L-shaped corner that is formed by virtue of the upper sheet being slightly smaller in dimension than the lower sheet (not shown in FIGS. 1-2). The solder glass for seal 4 is originally deposited around the periphery of the unit. The entire assembly including sheets 2, 3 and the solder glass material is then heated to a temperature of approximately 500xc2x0 C. at which the solder glass melts, wets the surfaces of the glass sheets, and flows by capillary action into the space between the sheets forming hermetic peripheral seal 4. This approximate 500 degree C. temperature is maintained for about an hour. After formation of edge seal 4 and of the seal around pump out tube 8, the assembly is cooled to room temperature.
As referenced above, high processing temperatures are required in the manufacture of conventional vacuum IG units. As mentioned in column 2 of the aforesaid ""395 patent, the conventional vacuum IG processing temperature is approximately xe2x80x9c500xc2x0 C. for 1 hour.xe2x80x9d Inventor Collins of the ""395 patent stated in xe2x80x9cThermal Outgassing of Vacuum Glazingxe2x80x9d, by M. Lenzen, G. M. Turner and R. E. Collins, that xe2x80x9cThe edge seal process is currently quite slow: Typically the temperature of the sample is increased at 200xc2x0 C. per hour, and held for one hour at a constant value ranging between 430xc2x0 C. and 530xc2x0 C. depending on the solder glass composition.xe2x80x9d
Unfortunately, these high temperatures and long periods of time at which such high temperatures are maintained in forming edge seal 4 are undesirable, especially when it is desired to use a tempered glass sheet in the IG unit. Tempered glass loses temper strength upon exposure to high temperatures as shown in FIGS. 3-4. Moreover, these high temperatures may have an adverse effect upon certain low-E coating(s) that may be applied to one or both of the glass sheets.
In xe2x80x9cCurrent Status of the Science and Technology of Vacuum Glazingxe2x80x9d, R. E. Collins and T. M. Simko, 1998, states that: xe2x80x9ca low temperature process to make a hermetic edge seal would overcome this difficulty; despite significant efforts, however, such a process has yet to be shown to be viable.xe2x80x9d
FIG. 3 is a graph illustrating how fully thermally tempered plate glass loses original temper upon exposure to different temperatures for different periods of time, where the original center tension stress is 3,200 MU per inch. The X-axis in FIG. 3 is exponentially representative of time in hours (from 1 to 1,000 hours), while the Y-axis is indicative of the percentage (%) of original tempering strength remaining after exposure. FIG. 4 is a graph similar to FIG. 3, except that the X-axis extends from 0 to 1 hour exponentially in terms of time.
Seven different curves are illustrated in FIG. 3, each indicative of a different temperature exposure in degrees Fahrenheit (F.). The different temperature curves/lines are 400xc2x0 F. (across the top of the FIG. 3 graph), 500xc2x0 F., 600xc2x0 F., 700xc2x0 F., 800xc2x0 F., 900xc2x0 F., and 950xc2x0 F. (at the bottom of the FIG. 3 graph). A temperature of 900xc2x0 F. is equivalent to approximately 482xc2x0 C., which is within the range utilized for forming the aforesaid conventional solder glass peripheral seal/joint 4. Thus, attention is drawn to the 900xc2x0 F. curve in FIG. 3, which is labeled by reference numeral 18. As shown, only 20% of the original temper remains after one hour at this temperature (900xc2x0 F. or 482xc2x0 C.). Such a loss of temper strength may result in certain window units not being able to pass safety codes set for environments where tempered glass is desirable. Thus, if thermally tempered glass sheets are to be used in an IG unit, it is undesirable to have to heat the unit to the processing temperatures required for conventional vacuum IG units having solder glass peripheral seals 4.
xe2x80x9cFabrication of Evacuated Glazing at Low Temperaturexe2x80x9d, by Griffiths, et al., discloses the provision of an indium wire around the edge of a vacuum IG unit for sealing purposes. Unfortunately, Griffiths, et. al. do not address detempering problems. A further disadvantage of Griffiths et. al. is that their provision of the indium seal in the form of a wire allows for substantial oxidation to occur which is disadvantageous in some applications.
It is apparent from the above, that there exists a need in the art for a vacuum IG unit, and corresponding method for making the same, where a hermetic seal may be provided between opposing glass sheets without having to heat the IG unit to the high processing temperatures (e.g. approximately 500xc2x0 C.) referenced above, thereby allowing thermally tempered glass to be used in vacuum IG units without losing substantial temper during the manufacturing process. This would also allow certain temperature sensitive low-E coatings to be utilized. There exists a further need in the art to provide an indium edge seal in a vacuum IG unit in a manner so that oxidation is minimized.
This invention will now be described with respect to certain embodiments thereof, accompanied by certain illustrations.
An object of this invention is to provide an edge seal for a vacuum IG unit, wherein the edge seal does not require processing temperatures greater than about 200 degrees C.
Another object of this invention is to provide an indium-inclusive edge seal for a vacuum IG window unit.
Another object of this invention is to provide a vacuum IG window unit having thermally tempered glass sheets, wherein processing temperatures utilized during manufacture of the IG unit allow the glass sheets to retain at least about 70% of their original tempering strength after the unit has been made, or after a seal-forming heating step has been completed.
Generally speaking, this invention fulfills the above described needs in the art by providing a method of making a thermally insulating glass product, the method comprising the steps of:
thermally tempering first and second glass substrates so that each substrate has an original tempering strength immediately after said tempering;
providing a sealing material between the first and second thermally tempered glass substrates;
heating the sealing material so that the sealing material bonds between the first and second substrates and forms a hermetic seal defining a low pressure space between the substrates;
and wherein the heating step includes heating the thermally tempered glass substrates to a temperature low enough and for a time short enough so that the first and second substrates retain at least about 70% of their original tempering strength after the heating step is completed.
In certain embodiments, the heating step includes heating the sealing material and the glass substrates to a maximum temperature of less than or equal to about 300 degrees C., most preferably to a maximum temperature of less than or equal to about 200xc2x0 C.
This invention further fulfills the above described needs in the art by providing a thermally insulating glass panel comprising:
first and second spaced apart thermally tempered glass substrates enclosing or defining a space therebetween;
an indium inclusive peripheral seal interconnecting the first and second spaced apart glass substrates so as to hermetically seal the space between the substrates; and
wherein the first and second glass substrates retain at least about 80% of their original tempering strength at a point in time after the peripheral seal is formed.
This invention further fulfills the above described needs in the art by providing an insulating glass (IG) window unit comprising:
first and second glass substrates spaced from one another so as to define a low pressure space therebetween, the low pressure space having a pressure less than atmospheric pressure so as to improve thermal insulating characteristics of the window unit;
at least one groove or notch defined in at least one of the glass substrates proximate an edge thereof;
a hermetic seal disposed between the substrates hermetically sealing the low pressure space from surrounding atmosphere so as to maintain pressure less than atmospheric pressure in the space, and sealing material forming the hermetic seal being at least partially located within the groove or notch.
In certain embodiments, the flexible hermetic seal has an elongation coefficient of at least about 10% so that the seal can stretch in a direction at least about 10% of its original size without losing its hermetic sealing function.